In recent years, portable electronic devices such as portable audio devices, mobile phones, and laptop computers have been widely used. From the viewpoint of energy saving or reduction of carbon dioxide emission, hybrid electric vehicles employing an internal combustion engine together with a driving force generated by electricity are becoming popular. With the spread of these devices and vehicles, there has been an increased demand for higher performance electricity storage devices used as electric power sources. Alot of research and development has been done particularly on non-aqueous electrolyte secondary batteries typified by lithium secondary batteries. Lithium secondary batteries are characterized by high voltages of 3 V or more and high energy densities. Lithium secondary batteries are required to have improved output characteristics, in particular, a pulse discharge characteristic that is an instantaneous high current characteristic, with their characteristic high energy densities being maintained.
In order to improve the output characteristics without a significant decrease in the energy densities, an approach of adding activated carbon to a positive electrode has been proposed (for example, see Patent Literatures 1 to 5). Activated carbon has an electric double layer capacitance formed by adsorption or desorption of anions or cations on its surface. Since charging of an electric double layer capacitance and discharging therefrom are rapid, the addition of activated carbon to a positive electrode may achieve both a high energy density and a high output power.
However, activated carbon has a very large surface area and the surface is very active. Therefore, while a lithium secondary battery including a positive electrode containing activated carbon is kept charged, decomposition of an electrolyte solution tends to proceed on the surface of activated carbon.
Furthermore, activated carbon has a very high adsorption capacity for trace amounts of gas components and water in the air. Therefore, such water adsorbed on activated carbon must be removed by a process such as vacuum drying or heat treatment to use activated carbon as a positive electrode material. This removal process takes a long time. Even through this removal process, it is difficult to remove adsorbed water completely. When a positive electrode of a lithium secondary battery is fabricated using activated carbon with adsorbed water remaining on its surface, problems such as evolution of gases in the battery and deterioration in charge/discharge cycle characteristics are likely to occur as the battery repeats charging and discharging.
Patent Literature 5 discloses a positive electrode made of activated carbon coated with a pseudo-capacitance type organic capacitor material. As such a pseudo-capacitance type organic capacitor material, a conductive polymer is disclosed.
However, since the capacitor of Patent Literature 5 uses activated carbon, it has the above problems. Furthermore, since conjugated electrons spread throughout the molecules of the conductive polymer that is proposed as a pseudo-capacitance type organic capacitor material, fewer electrons can be extracted therefrom. Therefore, such a polymer is not effective enough to improve the output characteristics.
Patent Literature 6 proposes a use of a carbon material capable of occluding anions in place of a part of a conductive material to increase the capacity. In this case, the carbon material capable of occluding anions contributes to the charge/discharge reactions and thus the capacity can be increased, but no improvement in the output characteristics of the resulting lithium secondary battery can be expected.
Patent Literature 7 discloses a secondary battery including an active material layer containing a lithium composite oxide and a radical compound, with different concentrations of the radical compound on the electrode surface side and the current collector side. However, since the radical compound in the active material layer is present disproportionately on the electrode surface side or the current collector side, it can contribute to a high output power only during either charging or discharging. Moreover, two or more coating solutions with different concentrations of the radical compound must be applied sequentially to the current collector to obtain such an active material layer, which makes it difficult to produce the secondary battery efficiently. Furthermore, Patent Literature 7 gives no detail description of the radical compound. Rather, it also gives compounds with different reaction potentials and different charge/discharge reaction mechanisms as specific examples of the radical compound. Therefore, the use of certain radical compounds or the use of such radical compounds in certain combinations with the lithium composite oxide results in insufficient output characteristics.